In the coating of hard substrates, such as plastics, there is frequently a desire for the coating to achieve a soft feel, where the coated surface, although feeling soft to the touch, is unimpaired in terms of the mechanical and chemical resistance in comparison with the conventional coatings.
Binders for these so-called soft feel coating materials preferably have segments of polymers having a low glass transition temperature; the crosslinking density must be high enough for there to be very little or no reduction in the mechanical and chemical resistance properties but low enough to allow the effect of the low glass transition temperature to be manifested macroscopically as well.
Two-component binders have been disclosed for soft feel coating materials based on hydroxyl-containing polyester urethanes, which are to be cured preferably with isocyanates but also, for example, with acid anhydrides or amino resins.
It has not proven possible to date to provide practicable one-component binders for soft feel coating materials. Although crosslinking with amino resins is able to take place at elevated temperature in the manner of a one-component system, the soft surface of the soft feel coating is adversely affected by the high temperatures required.
The known, polyisocyanate-based systems which cure at room temperature have the problems typical of isocyanates of limited pot life and poor acceptability from the standpoint of occupational hygiene. A further important disadvantage of isocyanate curing agents in conjunction with soft feel coating materials is the "aftercuring" which is observed. Because of the presence of water (which is itself able to react with the isocyanates), the polyfunctional isocyanates used for crosslinking have to be used in excess. Their reaction with water involves decarboxylation and formation of the corresponding amines, which in turn form ureas with isocyanates which have not yet reacted. This urea formation reaction takes place during the drying of the coating material and adversely affects the feel of the coating film, which is still soft directly following application. There was therefore a need to develop a binder for soft feel coating materials which durably retains this soft feel. A further object was to provide a one-component binder which cures at moderate temperature and yet imposes very little restriction on the pot life.